Abstract
In the evolutionary arms race between plants and herbivores, sophisticated mechanisms of indirect plant defense play a pivotal role. This study investigated the role of volatile organic compounds (VOCs) in attracting the parasitoid Ooencyrtus spp. to Aristolochia contorta, while also providing insights into the interactions among A. contorta, the herbivore Sericinus montela, and Ooencyrtus spp. in a tritrophic context. This study utilized field surveys, olfactometer experiments, and Gas Chromatography-Mass Spectrometry (GC-MS) analysis to investigate the role of VOCs. Field surveys showed a 54.6% egg parasitism rate, with quadrats containing A. contorta and larvae attracting more Ooencyrtus spp. than those with the plant alone. In olfactometer bioassays, Ooencyrtus spp. preferred leaves damaged by a pattern wheel simulating herbivore damage, with 46.8% choosing these leaves over undamaged controls. Leaves treated with larval saliva were similarly attractive, drawing in 48.7% of Ooencyrtus spp.; however, the difference in attraction between saliva-treated and untreated leaves was not statistically significant, suggesting saliva may not be central to Ooencyrtus spp. attraction. GC-MS analysis identified VOCs in damaged leaves, including hexyl acetate, cyclohexene, δ-cadinene, α-pinene, and β-caryophyllene, while saliva-treated leaves showed minimal amounts of exo-isocitral and β-pinene. Despite complex responses, our analysis suggests these saliva-induced compounds do not significantly boost Ooencyrtus spp. attraction. This finding implies that while the VOC response to damage and saliva application is multifaceted, serving multiple defensive functions, the amount of these saliva-induced compounds may be insufficient to substantially influence the behavior of Ooencyrtus spp. toward damaged leaves. Our results emphasize the role of VOCs in A. contorta's indirect defense mechanisms and contribute to understanding the ecological dynamics within plant-parasitoid-herbivore interactions. Moreover, our findings suggest new avenues for exploring the ecological and evolutionary roles of chemical signals, highlighting the complex interactions facilitated by these cues in plant defenses.